The detection of trace substances in air has many applications. An important application is the detection of substances which are being improperly carried by travellers or in containers, e.g. drugs, explosives or alcohol. It has been discovered, as disclosed in U.S. patent application Ser. No. 631,953 (now U.S. Pat. No. 4,580,440) of British Aerospace Public Limited Company, that many of such substances can be detected by collecting particulates which are emitted of such substances, and then analyzing the particulates. However the present inventors have discovered that the substances are often difficult to collect, and that special techniques are required to collect particulates and to collect vapours of low volatility. High efficiency collection is important because of the very low concentrations of the trace substances which are usually present.